In an inverter device that controls an electric motor, to reduce size, weight, and cost, and to improve reliability, a sensorless control method is suggested, in which a position sensor of a rotor, such as a resolver encoder, is not used.
For example, a method of estimating a rotor position by using voltage information generated by a no-load magnetic flux inter-linked to a coil, and a method of using harmonic current information generated due to a rotor salient pole while superposing a harmonic voltage are suggested.
In an inverter used for railroad or industrial applications, an initial estimation of a rotor position is necessary when activating an inverter from a coasting operation, or at the time of recovery from an instantaneous power failure, etc. In this case, a method of estimating the rotor position by controlling a switching pattern of the inverter upon reactivation to observe an electric current generated by short circuiting the coil, and a method of estimating the rotor position by suppressing an electric current generated by a magnet induced voltage to zero and then utilizing a feature quantity generated are suggested.
Furthermore, as a free run reactivation system for a permanent magnet synchronization motor (PMSM), a method of estimating the rotor position by switching an inverter circuit so as to output a non-zero voltage vector, and using one formula regardless of a motor speed is suggested.
In the case of controlling, for example, a synchronous reluctance motor (SynRM) as an electric motor, an inductance is significantly changed depending on the electric current to be supplied. In the PMSM, since the rotor includes a magnet, a center bridge of the rotor is always in a magnetic saturation state. Therefore, the change in inductance with respect to a change in an electric current (dynamic inductance) is small. In contrast, in the SynRM, since a magnetic saturation is enhanced at the center bridge of the rotor by supplying an electric current, the change in inductance with respect to the change in the electric current becomes greater than in the PMSM.
When performing the sensorless control, for example, in the case of using an inductance, as a parameter for control, when a magnetic saturation is not occurred, since a gap will occur between the actual motor parameter and the parameter for control, a rotation phase of the electric motor would not be accurately calculated, thereby disabling or destabilizing the sensorless control.